Low memory header type

About

The idea here is to create a header type and shared folder functionality that will no longer use CamelMessageInfo and CamelFolder but implements its own mmap to which TnyHeaderIface implementations will point for their data.

Why

Because in Camel you can't close the folder until the last summary-item (a header) isn't needed anymore. A folder consumes a lot memory (each CamelMessageInfo consumes ~120 bytes per count). query folders would use this header-type in such a way that yes the mmap of each folder with matches is loaded. But the implementation wouldn't use memory for each struct instance in the mmap. Only the headers that are really needed.

(Yes I know I need to outline this a little bit more)